Controlled spraying is a work practice that is very effective in reducing styrene emissions from the open molding process. It is an emissions source reduction method, which benefits the manufacturing process, plant personnel, the environment and reduces cost. Controlled spraying reduces styrene emissions by increasing material transfer efficiency through overspray minimization. Overspray has been found to be a major factor in styrene emissions. Transfer efficiency is the amount of material sprayed, compared to the amount retained on a mold surface. This is accomplished by minimizing spray gun atomization and reducing overspray loss. There are three elements of controlled spraying, which work together to reduce emissions:

• Operation of the spray gun at the lowest functional fluid tip pressure, which produces an acceptable spray pattern;
• Operator training that teaches proper spray gun handling techniques; and
• The use of close containment mold flanges to minimize spray off the mold edge.

Spray gun operator training is an important aspect of a controlled spraying program. It is important that spray applicators and production management know how to properly set-up a spray gun, and understand the proper methods of spray gun handling. The practical application of controlled spraying is based on training that specifies how these methods work, and provides methods for verifying the effectiveness of the techniques.

Controlled spraying should be used for all spray application of styrene based resins. This includes, polyester resins, vinyl ester resins, polyester gel coats, vinyl ester gel coats and other resins containing a volatile monomer. Controlled spraying is beneficial in all composites manufacturing processes, where materials are applied by atomized spray.

This work practice technique should be used as the standard manufacturing practice in open molding facilities, or in other cases, where atomized spraying is used.

Controlled spraying can provide a substantial reduction in styrene emissions, compared to typical uncontrolled spraying. Testing has demonstrated that controlled spraying can reduce gel coating emissions by up to 40%, and laminating resin emissions by up to 20%. Controlled spraying is a method of reducing styrene emissions that is universal for atomized spray application and provides benefits in a number of important areas.

There are a number of possibilities for styrene emissions reduction in the open molding process. These include the use of low styrene content resin or gel coat; the use of styrene suppressants; non-spray application such as flow coaters or pressure rollers; and controlled spraying. Low styrene resins and styrene suppressants may not be suitable for all circumstances. Likewise, non-spray application is not currently feasible with gel coat, and may not be suitable for all laminating resin applications. Controlled spraying, on the other hand, can be used in all circumstances where atomized spray application is required.

The emissions reduction, available from controlled spraying is a winning situation in all ways. The total quantity of styrene released to the environment, and required to be reported, will be reduced with the implementation of controlled spraying. Worker exposure to styrene will be reduced, and plant heating and ventilation flows may be able to be adjusted accordingly. Finally, the increased transfer efficiency offered by controlled spraying reduces material loss during the process. This has a positive impact on costs, in terms of quantities of materials consumed. Additionally, the reduction of overspray contributes to cleaner work areas and overall housekeeping efforts.

When using atomized spraying, approximately half of the total styrene emissions produced from the open molding process take place during the spray phase of the process. In the case of gel coating, 50% of the emissions occur during spraying and 50% during the curing process. With laminating resin spray application, about 55% of the emissions occur during spraying, 25% during the laminate roll-out phase, and 20% during the curing phase. The emissions from the spraying phase are a significant contributor to the total emissions.

Fluid stream atomization contributes to emissions during the spraying process. The greater the level of atomization (creating finer aerosol particle sizes) and the higher the fluid stream pressure, the more emissions will occur. Because a large portion of emissions occurs during the spray phase of application, spray gun pressure is of primary importance. The lowest pressure that produces an adequate spray pattern will produce the lowest emissions.

Another aspect of emissions involves the wet surface area of resin or gel coat. Once the material has covered the surface of the mold and surrounding area, emissions are a function of surface area evaporation. Therefore, the larger the surface area, the greater the evaporative loss. Since styrene evaporates from the surface only, the thickness of the laminate of gel coat film is not an issue once the material is in the static state on the surface. In this condition, the amounts of styrene lost from the surface of a thin film of overspray, and a thick laminate, may be almost the same. Therefore, reducing the surface area of overspray is a critical aspect of reducing surface area loss.

A mold has dimensions of 4 ft. X 8 ft. (or 32 ft2) plus a 6 in. perimeter flange
Emissions = 8 grams per ft2 during curing phase based on surface area calculations

In order to create a useful spray pattern, it is necessary to convert a pressurized stream of resin into an elliptical shape as it exits the spray gun fluid tip. This elliptical fluid stream is known as a fan pattern or spray pattern. Atomizing the fluid is to break the fluid stream into fine aerosol particle sizes, which converts the narrow high velocity fluid stream into a lower velocity shaped spray pattern. In many cases the spray pattern also provides the means for external mixing of an initiator (catalyst) component with the resin stream.

In order to achieve an acceptable fan pattern, a particular level of atomization is required. The required level of atomization will vary and is dictated by the characteristics of the rheology of the resin, resin temperature, spray gun type, required spray gun distance from the mold, and mold configuration. There is however, an ideal minimum level of atomization for each combination of factors.

Any additional atomization beyond that required level to form an adequate fan pattern, should be considered excessive. Over-atomization results in an increase in emissions from increased monomer evaporation and decreased transfer efficiency associated with enlarging the "wet footprint" of overspray.

The objective of minimizing atomization is to insure that atomization greater than required to produce an adequate fan pattern does not take place. This is accomplished by operating a spray gun at the lowest possible pressure at, which it develops a proper fan pattern.

Overspray is considered to be that resin which is deposited off the mold surface during the spraying process. This directly relates to transfer efficiency, which is the amount of material dispensed by the spray gun, compared to the amount deposited and retained on the mold.

Overspray has the effect of increasing the resin surface area by creating an enlarged "wet footprint", greater than the actual mold surface area. This increase in surface area contributes to an increase in emissions.

There are a number of types of spray guns, which may be used for gel coat or resin application. The general categories of spray equipment include:

• Conventional Air-Atomized
• Spray Guns;

  - Siphon Cup Gun

  - Gravity Feed Gun

  - Pressure Pot Gun

• High Pressure Airless Guns;
• Air-Assist Airless Guns; and
• High Volume Low Pressure (HVLP) Guns.

With an air-atomized spray gun, the resin is delivered to the spray tip at low pressure. As the low pressure fluid exits the spray nozzle, atomizing air is forced across the liquid stream causing it to form a fan pattern. Siphon cup gun draws liquids up the siphon tube using the venturi effect. The material is then pulled along with the atomizing air at the spray tip. The gravity feed gun has a material cup mounted on top of the spray gun. The liquid flows down into the gun head and is mixed with atomizing air at the spray tip. A pressure pot gun uses a pressurized container (pot) to force the fluid flow to the gun head. The fluid then exits the spray tip in a straight stream, which is then formed into a fan pattern by the atomizing air.

Air atomized spray guns are generally not used for production resin or gel coating application for several reasons. First, the flow rates produced by this type of gun are lower than required for most production applications. Second, air atomized spray guns produce the highest rate of emissions of all spray guns. The transfer efficiency (which is the amount of material sprayed compared to the amount which is retained on the mold) of air atomized spray equipment is very low. Because of this, material waste and emissions are higher then with other types of spray equipment.

Airless spray guns use a pump to deliver the resin coat to the fluid tip at high pressure. As the high-pressure stream exits the small fluid tip, the sudden reduction in pressure causes the fluid to atomize into a spray pattern. Developed in the 1960¹s, airless spray improves the transfer efficiency over the older air atomized application equipment. Airless spray tips usually require a fluid pressure of at least 1000 psi to produce an adequate fan pattern.

These current technology spray guns are a combination of airless and air atomized guns, drawing the benefits of both types. Air-Assisted Airless guns use a pump to deliver the resin to the fluid tip, but with much less pressure then an airless gun. The partially shaped fan pattern is then fully formed with the introduction of "shaping" air with the air-assist. The combination allows for reduced pressure operation with control over the fan pattern shape. Air-assisted airless guns produce higher transfer efficiency then airless guns with reduced emissions. The lower pressure spray also enhances gel coat quality.

HVLP spray guns have been used in the spray painting industry for some time. While similar to the air-assisted airless guns in many ways, these units operate with air atomizing pressures of 10 psi or less. The low air pressure is replaced with a high volume of airflow, which results in reduced emissions, and better transfer efficiency.

The most common type of resin pump is termed an air over fluid pump. An air driven piston drives a fluid piston, which forces the gel coat out to the spray gun at high pressure. The difference between the diameter of the air piston and the fluid piston is termed the pump ratio. Pump ratios usually range from about 11:1 up to 33:1. By multiplying the air-input pressure by the pump ratio the fluid pressure at the spray tip can be determined.

Example:

• Pump Ratio = 15:1 (15 psi of fluid pressure for every 1 psi of air pressure)
• Pump air pressure set at 40 psi
• Multiply: Pump Ratio x Pump Pressure Setting to determine the tip pressure
• 15 psi x 40 psi = 600 psi fluid tip pressure

Bob Lacovara is the Director of Technical Services for the Composites Fabricators Association

Spray Gun Set-Up & Pressure Calibration

Flow rate is the amount of material sprayed in a given period. The flow rate is controlled by the size of the spray tip, pump pressure and resin viscosity.

The spray tip orifice size is generally dictated by the required flow rate for the particular size and configuration of mold to be sprayed.

Large parts, requiring large amounts of resin, are usually sprayed with larger size tips. Smaller parts or parts that are more detailed are easier to spray with smaller size tips.

The viscosity or thickness of the gel coat will affect both the flow rate and fan pattern.

Viscosity is normally adjusted by the gel coat manufacturer, but is affected by temperature. Cooler material will be thicker and will reduce the flow rate. While warmer gel coat is lower in viscosity and flows at a higher rate.

Determining the Proper Spray Gun Pressure

Determining the ideal pump pressure for a specific combination of material and equipment is an important element of controlled spraying. The goal is to apply resin or gel coat at the lowest level of atomization, which produces a workable spray pattern.

Sometimes operators feel they have to turn up the pressure to get an adequate flow rate. The proper method is to maintain minimum pressure and to increase the size of the spray tip to match the required delivery rate.

It is always an advantage to spray at the lowest possible pressure. The lowest pressure will:

• Reduce Styrene Emissions;
• Minimize overspray;
• Create better working conditions;
• Enhance catalyst mixing;
• Reduce material usage;
• Reduce equipment wear;
• Reduce high pressure hazards;
• Reduce static charge build-up; and
• Increase product quality.

The proper spray gun pressure calibration procedure is very straightforward and is appropriate for all production spray guns.

1. Verify that the resin is the correct temperature, and has been properly mixed within the manufacturers specified period.

2. Verify that the spray tip is in good condition and suitable in flow rate range and fan pattern width for the given job.

3. Aim the spray gun at a disposable surface on the floor, maintaining a distance of 12" to 18" and perpendicular to the floor.

4. Turn the pump pressure down to zero and pull the trigger.

5. Slowly begin to increase the pressure in 10 psi increments until the fan pattern is adequate.

6. Record this pressure in the spray gun set-up log.

7. Do not increase the pressure past this point. The result will be over-atomization and increased overspray, and poor transfer efficiency.

The size of a spray pattern results from a unique combination of orifice size, tip angle and resin flow characteristics. The required fan pattern width is specific to the size and configuration of the part being sprayed. The size of the spray pattern should match the spraying requirements. For example, a large flat part requires a wide fan pattern. A small part or one with a complex shape may require a narrow fan pattern.

There is, however, one trait all spray patterns have in common; a symmetrical shape where the material is distributed evenly across the length and width of the spray pattern.

Fan patterns develop from a round straight stream of resin at very low pressure to elongated, but irregular shapes as the pressure increases. As the pressure reaches the perfect point for that combination of factors, a symmetrical elliptical shaped spray pattern develops. If the pressure is increased past this point, the width of the pattern might increase slightly, but there will be no improvement in the symmetry.

Operator spraying technique is an essential factor in reducing emissions as well as producing a high quality work. There are basic elements of spraying technique of which contribute to effective application of resin or gel coat.

Where you aim the spray gun is important. The object of controlled spraying is to minimize overspray. Always attempt to hold the gun perpendicular to the surface. As the spray gun assumes a lower angle in relation the surface, overspray will increase.

Spray the perimeter of the mold first, while maintaining overspray within the boundary of the close containment flange. When spraying the interior of the mold, work out to the material previously applied to the perimeter and stop short of going off the mold edge.

Always begin by spraying the section nearest you. The reason for this is to prevent a momentary off-ratio burst of material from spraying on the mold surface. By starting with the closest area of the mold and working outward, you will minimize the likelihood of raw catalyst falling on exposed mold surface and possibly damaging the mold.

Avoid triggering the gel coat gun on and off as you would a paint spray gun. In addition the trigger should be "full-on or full-off" to maintain the proper material ratio. Do not "throttle" the gun with a partially open trigger.

Always attempt to keep the fan pattern at right angles (perpendicular) to the mold surface and avoid an arching motion with the gun. Follow the contour of the mold as closely as possible. Avoid spraying at a low angle. It will be difficult to control overspray and the material thickness will taper off further away from the gun. In the case of an arcing spray movement, the change in spray angle at the end of the stroke will make overspray difficult to control.

Do not assume you can apply the proper thickness by feel or experience. There are a number of variables, such as tip size and condition, pump pressure, viscosity and temperature that can affect the delivery rate. Because of this it is essential for even experienced operators to mil gauge every part.

Applying the right film thickness is a function of time and motion. The spray gun puts out a specific amount of material in a given amount of time. How much material an area is a matter of how fast the gun covers an area. The operator must concentrate on maintaining a constant speed throughout the application.

It is best to spray an area about as large as a comfortable arm swing. Avoid pivoting the gun with the wrist and do not bounce the spray pattern. The proper technique is to use smooth long strokes, while keeping track of spray bands.

Controlled spraying is one of those rare situations in FRP production where there is no downside. All of the effects of this technique are beneficial. In addition, controlled spraying is "do-able" in every plant at little cost. You need to make controlled spraying the standard in your plant, to prepare for the demands of tighter regulation and to upgrade your production process.

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